1. Field of the Invention
The invention relates to a conveying method and apparatus for conveying large quantities of dry bulk materials through a material conveying pipe.
2. Description of Related Art
Pneumatic conveying systems are designed to move large quantities of particulate material in an air (or other inert gas) medium through a material conveying pipe. Pneumatic conveying systems are of two general types: dilute phase systems and dense phase systems. Typically, the type of the particulate material to be conveyed determines what type of pneumatic conveying system to utilize.
In dilute phase conveying systems, particulate material is metered into the conveying pipe from a hopper or other holding device. A standard blower delivers a high volume of air (or inert gas) at a low pressure, typically not exceeding 15 psig, that is used to convey a low volume of particulate material. The stream travels at high velocities, typically between 4,000 to 6,500 ft/min, in order to keep the light particulate product load in suspension in the flowing gas stream. In dilute phase conveying systems, the particulate material being conveyed is usually very free flowing, and the solids loadings is relatively low, typically on the order of 5 to 15 pounds of particulate material per pound of gas. In general, dilute phase systems are typically used to convey non-abrasive and non-fragile materials that have low densities, such as flour, potato starch, cornstarch, calcium carbonate, hydrated lime, activated carbon, and zinc oxide.
Dense phase conveying systems are generally characterized by lower inert gas velocities and much higher conveying pressures operating in a batch mode. Dense phase conveying systems are typically used to convey abrasive and/or friable material, such as silica sand, fly ash, alumina, carbon black, cocao beans, hazel nuts, corn, plastic pellets, and puffed rice. In such systems, a containment vessel is filled (typically by gravity feed) with the particulate material, sealed, and then pressurized to the desired high pressure. Subsequent release of the pressure discharges the material and propels it along the conveying pipe to its intended destination. The stream travel at velocity typically between 1000 and 3000 ft/min, and the conveying pressure may be as high as 60 psig. In contrast to dilute phase conveying systems, dense phase conveying systems utilize higher ratios of particulate material to the amount of gas used and thus have higher solids loading. Because air compressors and ASME code equipment must be used in order to achieve and withstand the high conveying pressures, dense phase systems have dramatically increases the capital and operating cost of the system compared to that of dilute phase systems.
Dense phase conveying systems suffer from other drawbacks. In dense phase conveying systems, the pressure containment vessel must be closed and the pressurized gas vented off before the pressure containment vessel can be recharged for the next batch of particulate material to be conveyed. Alternatively, the pressure containment vessel is returned to the necessary lower pressure for refilling by letting the high pressure air (or inert gas) dissipate down the material conveying line. The foregoing dense phase conveying system is typically referred to as a “batch” dense phase conveying system. Thus, a major disadvantage associated with batch dense phase conveying systems is that there is a significant amount of non-product conveying time involved while the pressure containment vessel is venting, being refilled, and pressurized. Typically, the system is only operating about 60% of the time. Another disadvantage is that, because of the significant amount of non-conveying time, the conveying pipe size must be relatively large to compensate for the lost time. Another disadvantage is that the last portions of the particulate material leaving the pressure containment vessel tend to accelerate rapidly as the material conveying pipe is blown free and the high pressure gas seeks to dissipate down along the material conveying pipe. The increased acceleration of the particulate material being conveyed often causes a significant amount of product degradation to the last portion of the particulate material being conveyed.
So-called semi-dense phase conveying systems or “blow pot” systems are also known in the art. In general, these conveying systems are a combination of dense phase and dilute phase systems. The blow-pot system has high solids loadings and operates in a batch mode like dense phase systems. Because the lower pressures are utilized, the equipment does not need to meet ASME code requirements and generally utilizes gas blowers instead of air compressors like the dilute phase systems. However, blow-pot systems still run in batch mode and have significant non-conveying time.
More recently, continuous dense phase conveying methods have been proposed, such as those described in Wallace, U.S. Pat. No. 5,407,305. In general, a high pressure rotary airlock valve is utilized to feed the particulate material into the conveying pipe using high pressure gas, typically on the order of 45 to 60 psig. Because of the high pressures used to move material, the equipment utilized must meet ASME code requirements, and air compressors must be utilized, resulting in high capital costs.